A few of these Pokémon, as a result of new evolutions, become a part of evolutionary families with branched evolutions. Those Pokémon which evolve into a Pokémon of a previous generation, instead of from it, are specifically baby Pokémon.
Generation IV introduced the most evolutionary relatives to Pokémon from previous generations, numbering 29; Generation II introduced 19, Generation III two and Generation VI only one. Neither Generation V nor Generation VII introduced any new cross-generational evolutions (not counting regional variants).
Roselia is the only Pokémon to have both a baby and an evolution (Budew and Roserade, respectively) introduced in the same generation.
Sylveon is the only Generation VI cross-generational evolution.
Porygon is the only Pokémon to have an evolution introduced in a later generation that was later able to evolve once again.